[HASKELL] Generated types do not produce/consume JSON according to spec

Description

In a Swagger spec file, when specifying eg. "payment_data" as a field name in an object, the resulting Haskell type produces (and consumes) JSON with a field name of "PaymentData". So eg. "channel_value_left" becomes "ChannelValueLeft". This breaks compatibility with the client library.

Swagger-codegen version

master @ a3610f1c12e5d7912cf2e9f16dff2152ef40291b

Swagger declaration file content or url

https://gist.github.com/runeksvendsen/305a3018fe48723998851419c2e4393d

Command line used for generation

$ java -jar swagger-codegen/modules/swagger-codegen-cli/target/swagger-codegen-cli.jar generate -l haskell -i swagger.yaml
$ java -version
    java version "1.8.0_92"
    Java(TM) SE Runtime Environment (build 1.8.0_92-b14)
    Java HotSpot(TM) 64-Bit Server VM (build 25.92-b14, mixed mode)

Steps to reproduce

wget https://gist.githubusercontent.com/runeksvendsen/305a3018fe98851419c2e4393d/raw/5d022bdef0355128c54d2235ea986aa84efa3e3e/swagger.yaml
java -jar swagger-codegen/modules/swagger-codegen-cli/target/swagger-codegen-cli.jar generate -l haskell -i swagger.yaml
cat lib/BitcoinPaymentChannelRESTProtocol/Types.hs

Suggest a Fix

Perhaps add a config option like the modelPropertyNaming option that typescript-node has, which allows choosing word separation semantics ('camelCase', 'PascalCase', 'snake_case' etc.).

Start by running the provided Java generation command with the linked swagger.yaml, then inspect lib/BitcoinPaymentChannelRESTProtocol/Types.hs. Compare the generated JSON field names with the Swagger names, especially payment_data and channel_value_left; done means the generated Haskell types produce and consume the specified names.